Wrist device for a prosthetic limb

ABSTRACT

A wrist device for a prosthetic limb is provided. The device ( 1 ) comprises a base member ( 3 ) connectable to the wearer of the device, and a support member ( 13 ) connectable to the limb. The support member ( 13 ) is pivotably connected to the base member ( 3 ) such that the support member can pivot about a pivot axis (A) relative to the base member. A damping mechanism is located between the base ( 3 ) member and the support member ( 5 ). The damping mechanism comprises a pinion ( 47 ) connected to the support member ( 5 ) and rotatable about the pivot axis (A) relative to the base member ( 3 ). A rack ( 35 ) is engaged with the pinion ( 47 ) such that rotational motion of the pinion causes a linear motion of the rack, and at least one biasing member ( 41 ) extends between the base member ( 3 ) and the rack. The biasing member ( 41 ) biases the rack ( 35 ) and support member ( 13 ) into a neutral position. A prosthetic limb incorporating the wrist device is also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase Application of PCTInternational Application Number PCT/GB2015/052780, filed on Sep. 24,2015, designating the United States of America and published in theEnglish language, which claims the benefit of priority to Great BritainPatent Application Number GB 1417541.8, filed on Oct. 3, 2014. Thedisclosures of the above-referenced applications are hereby expresslyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to the field prosthetic limbs. Morespecifically, the present invention is a wrist device which allows aprosthetic limb to be manipulated relative to the wearer of the limb.

BACKGROUND OF THE INVENTION

Wrist devices for the manipulation of prosthetic limbs are known. Insuch devices, a prosthetic limb such as a prosthetic hand or digit isfixed to a support member which is pivobuttonly coupled to a basemember. The base member is attached to a socket located on the wearer'sresidual limb or stump. The support member and hand can pivot relativeto the base member in order to move the hand/digit into differentpositions.

An example of one such device is disclosed in US2007/0260328, in which aprosthetic hand and support member can pivot in a flexion/extensionplane relative to a base member. A gear segment attached to the supportmember has a number of indentations which are engaged by resilientretention elements which hold the support member and hand in certainpositions relative to the base member. The wearer pulls or pushes theprosthetic hand in the flexion/extension plane in order to move the handto a new position. As this takes place the retention elements are forcedout of their current indentations in the gear segment and will enter theadjacent indentations as the gear segment rotates. In addition to theretention elements a sliding locking device can be slid in and out ofengagement with slots in the gear segment in order to lock the hand in aparticular position. The locking device is biased into a locked positionby a spring.

A similar arrangement is disclosed in U.S. Pat. No. 7,144,430, whichalso discloses an arrangement in which the hand and support member canbe manipulated in both the flexion/extension and adduction/abductiondirections about a form of universal joint. Torsion springs are providedwhich bias the hand into a neutral position. One or more spring-loadedlocking pins engage the hand to lock it into positions along one or bothof the flexion/extension and adduction/abduction axes.

In arrangements such as those referred to above, the prosthesis wearercan find it difficult to overcome the biasing forces of the resilientmembers or springs which hold the hand in position or force the handtowards a given position. Consequently, repeated movements of the handabout the wrist device can lead to fatigue in the wearer. In addition,the biased locking arrangements which lock the hand in a given positioncan be cumbersome and awkward for a wearer to actuate with theirremaining natural hand, and even more so if they have another handprosthesis.

It is an object of the present invention to obviate or mitigate one ormore of these disadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided wristdevice for a prosthetic limb, the device comprising:

-   -   a base member connectable to the wearer of the device;    -   a support member connectable to the limb, the support member        being pivotably connected to the base member such that the        support member can pivot about a pivot axis relative to the base        member; and    -   a damping mechanism located between the base member and the        support member, the damping mechanism comprising:        -   a pinion connected to the support member and rotatable about            the pivot axis relative to the base member;        -   a rack engaged with the pinion such that rotational motion            of the pinion causes a linear motion of the rack; and        -   at least one biasing member extending between the base            member and the rack, the biasing member biasing the rack and            support member into a neutral position.

The device may comprise first and second biasing members, with each ofthe first and second biasing members extending between the base memberand the rack and biasing the rack and support member into the neutralposition from substantially opposite directions.

The first and second biasing members may comprise first and secondcompression springs located between the rack and the base member,wherein the first spring biases the rack in a first direction and thesecond spring biases the rack in a substantially opposite seconddirection.

The compression springs may be partially housed within the rack, whereina first end of each spring is engaged with an internal surface of therack and a second end of each spring is engaged with the base member.

The base member may be provided with first and second adjustment memberswhich can adjust the biasing force of the first and second springs,respectively.

The device may further comprise a locking mechanism for selectivelypreventing movement of the support member relative to the base member,the locking mechanism comprising an actuator which is moveable betweenlocked and unlocked positions, the actuator including a first lockingelement which prevents movement of the pinion when the actuator is inthe locked position.

The actuator may comprise an actuator shaft extending through the pinionalong the pivot axis, and first and second actuator buttons at oppositeends of the shaft, wherein the actuator can move axially along the pivotaxis relative to the base member and pinion when the first or secondbutton is pressed, one of the first and second buttons incorporating thefirst locking element.

The device may further comprise a pinion shaft which is rotatablysupported on the base member and rotatably coupled to the pinion,wherein the pinion shaft has a second locking element at one endthereof, the first and second locking elements engaging to preventrotation of the pinion when the actuator is in the locked position.

The button which incorporates the first locking element may have abottom surface which engages an upper surface of the base member whenthe actuator is in the locked position, thereby preventing relativerotation between the actuator and the base member.

At least one of the base member and actuator may include one or moremagnets, the or each magnet attracting the actuator towards the lockedor unlocked position when the actuator is within a predetermineddistance of the base member.

The base member and first and second actuator buttons may includemagnets such that each button is attracted to the base member when itcomes within a predetermined distance of the base member.

The locking mechanism may further comprise a cover member having anopening through which the support member extends, the cover memberenclosing the locking mechanism and engaging the first and secondactuator buttons such that axial movement of the cover member willresult in a corresponding movement of the actuator.

The device may further comprise at least one flexible covering locatedbetween the pinion and the opening in the cover member so as to preventingress of contaminants into the device.

The base member may include a quick release coupling for connecting thedevice to the wearer.

According to a second aspect of the invention there is provided aprosthetic limb comprising a wrist device according to the first aspectof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described,by way of example only, with reference to the following drawings:

FIG. 1 is a perspective view of a wrist device for a prosthetic limb;

FIG. 2 is an exploded view of the device shown in FIG. 1;

FIG. 3 is a perspective view of the wrist device of FIGS. 1 and 2 whenpartly assembled; and

FIGS. 4 and 5 are perspective and top views respectively of the deviceof FIG. 1 with certain components removed for illustrative purposes.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a wrist device 1 for a prosthetic limb (not shown), whichmay be a prosthetic hand, for example. The wrist device 1 comprises abase 3, which is provided with attachment means for attaching the base 3to a socket (not shown) on a patient's residual limb or stump. In theillustrated embodiment, the attachment means is a quick releaseconnection in the form of a spring-loaded ball bearing arrangement 5, inwhich a plurality of ball bearings 7 are circumferentially arrangedaround the outer surface of the base 3. The ball bearings 7 are biasedradially outwards by biasing means (not shown) so that each ball 7 ispartially exposed through apertures 9 in a retaining ring 11 which sitson the outer surface of the base 3. The socket has an equivalent femalerecess with a plurality of indentations corresponding to the number ofball bearings 7. Consequently, as the base 3 is inserted into the femalerecess in the socket the ball bearings 7 are pressed radially inwardsagainst the force of the biasing means, and when the ball bearings 7meet the indentations of the socket they are pressed radially outwardsby the biasing means in order to locate in the indentations. In thismanner, the base 3 will be connected to the socket until such time arethe wearer applies sufficient force to the base by twisting and/orpulling in order to overcome the force of the biasing means anddisconnect the base 3 from the socket.

A support member, or plate, 13 is pivotably connected to the base 3 sothat the plate 13 may pivot about a pivot axis A relative to the base 3.The plate 13 is provided with a number of apertures 15 which allow theprosthetic limb to be attached to the plate 13. Pivoting about the pivotaxis A permits the plate 13 to move in directions which replicateflexion and extension movements of a natural wrist relative to theforearm. These respective movements are illustrated by arrowheads F andE in FIG. 1.

The device 1 further comprises a locking mechanism which the wearer canactuate in order to selectively lock the plate 13 in a particularposition relative to the base 3. The only component of the lockingmechanism which is visible in FIG. 1 is a cover member 17 which can bemoved axially along the axis A between positions which correspond withlocked and unlocked states of the locking mechanism. Further details ofthe locking mechanism and the manner in which it operates will bedescribed in more detail below.

The components which permit the pivoting of the support plate, and alsothe components which make up the aforementioned locking mechanism, canbe viewed in FIGS. 2, 4 and 5. A generally circular mount 19 forming apart of the base 3 is provided, the mount 19 having a pair of upwardlyextending lugs 20,21 on opposing sides of the mount 19. Each lug 20,21includes an aperture 23 through which, in use, the pivot axis A extends.A cover plate 25 is provided between the mount 19 and base 3 in order tocover the base. One or more shims 27 may be provided on an upper surface68 of the mount 19 so as to adjust the height of that upper surface 68,the reasons for which will be explained in more detail below. The mount19 is attached to the base 3 by a plurality of threaded screws 29.

Each lug 20,21 is provided with a pair of openings 31 which extendthrough the lug 21 below its respective aperture 23. Alternatively, eachlug 20,21 may be provided with a pair of recesses which open on theouter faces of the lugs 20,21. In other words the recesses open on thefaces of the lugs 20,21 which face away from one another. A mount magnet33 is fixed into each opening 31 or recess using glue or a similarbonding agent, so that the mount magnets 33 are exposed at least on theouter faces of the lugs 20,21.

A rack 35 is located on the base 3 within a central opening 28 in themount 19. Both the rack 35 and opening 28 are generally square orrectangular in shape, but the opening 28 is wider than the rack 35 in adirection substantially transverse to the pivot axis A so that the rack35 may slide transversely within the opening 28 along an axis which issubstantially perpendicular to the pivot axis A. The rack 35 includes apair of hemi-cylindrical recesses 37 which extend in the transversedirection relative to axis A. Each end of the rack 35 is provided with aplurality of upwardly extending teeth 39 which run along the ends of therack in the transverse direction.

A biasing means in the form of a compression spring 41 is located ineach of the hemi-cylindrical recesses 37 in the rack 35. Each recess 37is open on one side of the rack 35 and is closed on the opposing side ofthe rack 35, with the two recesses 37 open on opposing sides of the rack35. As a result, one end of each spring 41 lies against an internalsurface of the rack 35 defined by the closed end of its respectiverecess 37 and, when the rack 35 is located in the opening 28, the otherend of each spring 41 lies against a surface of the mount 19 adjacentthe opening 28. Stop washers 43 are placed in the opposing ends of eachspring 41 to assist with the engagement of the springs 41 with theirrespective surfaces. With this arrangement, the springs 41 hold the rack35 in equilibrium within the opening 28 in the mount 19, with thebiasing forces created by the springs 41 pushing in substantiallyopposite directions and being substantially equal and balanced. Finetuning of the biasing forces and balance of the rack 35 can beundertaken by way of first and second adjustment screws 45 which extendthrough either side of the mount 19 and engage an end of theirrespective first and second springs 41.

FIG. 2 also illustrates the components of the device 1 which provide thepivoting arrangement and the locking mechanism. A pinion 47 is providedwhich has a gear element 49 at either end thereof. Adjacent each gearelement 49 is an upwardly facing mounting surface 51 to which thesupport plate (not shown in FIG. 2) is fixed via a pair of threadedapertures 53 in each mounting surface 51. A bore 55 extendslongitudinally through the pinion 47 and in use the pivot axis A willextend through the bore 55. Each gear element 49 has a plurality ofdownwardly projecting gear teeth 50 which in use engage with theupwardly projecting teeth 39 of the rack 35. Consequently, when thedevice 1 is assembled rotation of the pinion 47 about pivot axis A willresult in a linear motion of the rack 35 perpendicular to the axis Awithin its opening 28 in the mount 19.

During assembly of the device, the pinion 47 is placed between the firstand second mount lugs 20,21 and a pinion shaft 57 is extended throughthe lug apertures 23 and pinion bore 55. Bearings 59 are located on theshaft 57 between each end of the pinion 47 and its respective lug 20,21.A pair of grub screws 61 connect the pinion 47 to the pinion shaft 57via apertures in both components so that they rotate as one element.

The pinion shaft 57 is hollow and includes a locking element in the formof a toothed wheel 58 attached to one end thereof such that when theshaft 57 is in position the toothed wheel 58 is located outside thesecond mount lug 21. A first actuator button 63 is provided which has anactuator shaft 65 attached to it. During assembly, the first button 63and the actuator shaft 65 are brought in from outside the first mountinglug 20 and the shaft 65 is fed through the aperture 23 in the first lug20, the pinion shaft 57 and the aperture 23 in the second lug 21 so thatthe free end of the actuator shaft 65 projects from the toothed wheel 58end of the pinion shaft 57. A second actuator button 67 is thenconnected to the free end of the actuator shaft 65. As a result, thefirst and second actuator buttons 63,67 and the actuator shaft 65running between them can now slide as an actuator axially along thepivot axis A relative to the pinion 47, pinion shaft 57 and mount 19.Both the first and second actuator buttons 63,67 have flat bottomsurfaces 64,66 which engage with an upper surface 68 of the mount 19whenever one of the buttons 63,67 is pushed inwards towards the mount19, thereby preventing rotation of the buttons 63,67 and actuator shaft65 relative to the mount 19.

Axial movement of the actuator arrangement is limited by the buttons63,67 coming into contact with the outer faces of the first and secondmount lugs 20,21. Although not visible in FIG. 2, the inner face of thesecond actuator button 67 has a locking element in the form of a toothedrecess which corresponds to the general shape of the toothed wheel 58 onthe end of the pinion shaft 57. Either the teeth of the toothed wheel 58or those of the toothed recess may have chamfered leading edges toassist with the engagement of the wheel 58 and recess when axialmovement of the actuator brings the second actuator button 67 intocontact with the toothed wheel 58. When the second button 67 is pressedas far as possible in towards the mount 19, the toothed wheel 58 willlocate in the locking recess of the second button 67 and rotationalmotion of the pinion 47 and pinion shaft 57 relative to the buttons63,67 will be prevented. Furthermore, as the flat bottom surface 66 ofthe second button 67 will be engaged with the upper surface 68 of themount 19, rotation of the actuator buttons 63,67 and their shaftrelative to the mount 19 is also prevented so the rack and pinionarrangement will be locked in its present position.

Button magnets 69 are located in recesses or openings in the buttons63,67 so that the magnets 69 are partially exposed on at least the innersurfaces of each button 63,67. In other words, the magnets 69 arepartially exposed on the surfaces of the buttons 63,67 which will comeinto contact with the mount lugs 20,21. The magnets 69 in the first andsecond buttons 63,67 and the mount magnets 33 fixed in the mount lugs20,21 attract one another so as to assist in sliding the buttons 63,67towards and away from the locking position.

Once assembly has been completed of the aforementioned components, thecover member 17 can be placed over the sliding lock mechanism in orderto make it as easy as possible for a wearer to operate the lock. Theunderside of the cover member 17 is provided with a pair of slots 18,one of which is visible through an opening 16 in the cover 17 in FIG. 2.These slots 18 receive the actuator buttons 63,67 in a snap fitting suchthat the cover 17 is then detachably secured to the locking mechanism,and axial movement of the cover 17 along the pivot axis A will move theactuator in and out of the locking position.

With the cover 17 attached to the locking mechanism the device 1 willappear as shown in FIG. 3. A pair of flexible coverings 70 are placedeither side of the opening 16 in the cover 17 so that the coverings 70lie between the cover 17 and the components of the device containedtherein. The support plate 13 can then be attached to the mountingsurfaces 51 on the pinion 47 via screws 72 extending through thecorresponding apertures 74 in the support plate 13. With the supportplate 13 attached to the pinion 47, the coverings 70 protect the innerworkings of the device from the ingress of dirt and other contaminantsthrough the cover opening 16, and also improve the appearance of thedevice with the majority of the components hidden.

To operate the device, the wearer can lock or unlock the device eitherby manipulating the cover 17 with their other hand, or else by gentlyknocking one end of the cover against a surface. The magnets 33, 69 inthe mount lugs 20,21 and actuator buttons 63,67 assist in moving thedevice between the locked and unlocked positions as they draw theactuator buttons 63,67 towards their respective lugs 20,21 when theycome within a certain distance of the lugs 20,21. The magnets alsoprovide a sufficient attraction force that the cover cannot be moved outof its present position without a reasonable force being applied to thecover. This is especially beneficial when the device is locked in agiven position, so that the chances of the device being accidentallyunlocked are reduced.

When in the unlocked position, the pinion 47 and support plate 13 arefree to rotate about the pivot axis A relative to the base 3. At thesame time, the springs 41 provide a damping force such that any forcesapplied to a hand attached to the plate 13 such as, for example, causedby the hand accidentally knocking against a surface, are not transmittedthrough the device to the socket and residual limb of the wearer.Furthermore, the springs 41 provide balanced biasing forces either sideof the rack 35 which will return the plate 13 and limb attached theretoto a neutral position when the device is unlocked and no forces arebeing applied to the hand.

The neutral position is a rest or zero position assumed by the rack 35and plate 13 when no external forces other than those of the biasingsprings 41 are being applied.

When the wearer wishes to place their prosthetic hand in a givenposition in the flexion/extension plane, they first manipulate the handinto the desired position using their other hand or by pressing the handagainst a surface in order to overcome the biasing forces of the springs41. Then, whilst maintaining the desired position of the hand, they thenmove the cover 17 towards the locked position. As described above,moving the cover 17 to the locked position brings the recess in thesecond actuator button 67 into contact with the toothed wheel 58 on theend of the pinion shaft 57 and also brings the bottom surface 66 of thebutton 67 into contact with the upper surface 68 of the mount. Thus, thebutton 67 cannot itself rotate and also holds the pinion shaft againstrotation at the same time. One or more of the shims 27 can be placedupon the upper surface 68 of the mount in order to eradicate play orbacklash between the teeth of the toothed wheel 58 and correspondingtoothed recess in the second actuator button 67.

FIGS. 4 and 5 show the fully assembled device in the locked position,with the support plate and cover removed for illustrative purposes.

The wrist device of the present invention provides a damping arrangementin which the support plate and limb attached thereto biased towards arest or neutral position. As the biasing forces applied are balanced thewearer does not need to apply a large force to the limb in order toovercome those biasing forces. The wearer will therefore not experienceas much fatigue using this device as is caused by repeated movements ofthe prior art devices such as those described in the introduction tothis specification. Furthermore, partially locating the biasing meanswithin the rack allows the device to be more compact than known wristarrangements.

The locking mechanism of the present invention provides a linearactuator which can be shuttled between locked and unlocked positionssimply by the wearer pressing on a given end of the actuator, or indeedby pressing one end of the actuator against a surface. No biasing forcesneed to be overcome by the wearer in order to unlock the device, againlimiting the fatigue which a wearer will encounter from repeatedlymoving and locking the device in different positions. When present,magnets placed in at least one of the base member and actuator ensurethat the actuator cannot be moved from the locked position without apredetermined force being applied to the actuator, but also assist thewearer in sliding the actuator into the desired position.

Although the preferred embodiment of the device incorporates a pair ofbiasing members, the balanced biasing force applied to the rack mayalternatively be provided by way of a single biasing member (e.g. a coilspring) connected between the base and rack and arranged so movement ofthe rack in either direction perpendicular to the pivot axis will beresisted by the biasing member. Furthermore, whilst springs such ascompression springs are the preferred biasing members, alternativebiasing members such as resilient elastomer rods, for example, may beused instead.

Whilst the biasing members are preferably partially located in the rackfor compactness of the device, they may be placed outside the rack andapply their forces in opposing directions to outer surfaces of the rack.

The locking mechanism may comprise an actuator with a single button forsliding the actuator into a locking position. Such an actuator may havea single locking element which engages the pinion to prevent rotationalmotion thereof. The pinion need not have a pinion shaft, but instead maybe rotatably supported by bearings directly on the actuator shaft.

Whilst magnets are preferably provided on each of the base member andactuator, the device may only have one or more magnets on one of thebase member and actuator.

The device may be provided without a cover member, wherein operation ofthe actuator would be achieved by directly pressing the actuator buttonson either end of the actuator.

The base member may be provided with an alternative arrangement forconnecting the base to the wearer. For example, the base may be providedwith a bayonet fitting which would engage with a corresponding fitmenton a socket worn by the user.

Whilst the preferred embodiment of the present invention describes theuse of the device with a prosthetic hand, it should be understood thatthe invention may also be used with other prosthetic limbs. For example,the device may support a limb comprised simply of one or more digits ona support chassis rather than a complete hand.

These and other modifications and improvements may be incorporatedwithout departing from the scope of the invention.

The invention claimed is:
 1. A wrist device, the device comprising: abase member connectable to the wearer of the device; a support memberconnectable to the limb, the support member being pivotably connected tothe base member such that the support member can pivot about a pivotaxis relative to the base member; and a damping mechanism locatedbetween the base member and the support member, the damping mechanismcomprising: a pinion connected to the support member and rotatable aboutthe pivot axis relative to the base member; a rack extending along anaxis and engaged with the pinion such that rotational motion of thepinion causes a linear motion of the rack along the axis; and at leastone biasing member extending between the base member and the rack, thebiasing member biasing the rack and support member into a neutralposition; a locking mechanism for selectively preventing movement of thesupport member relative to the base member, the locking mechanismcomprising an actuator which is moveable between locked and unlockedpositions, the actuator including a first locking element which preventsmovement of the pinion when the actuator is in the locked position;wherein the actuator comprises an actuator shaft extending through thepinion along the pivot axis, and first and second actuator buttons atopposite ends of the shaft, wherein the actuator can move axially alongthe pivot axis relative to the base member and pinion when the first orsecond button is pressed, one of the first and second buttonsincorporating the first locking element; wherein the button whichincorporates the first locking element has a bottom surface whichengages an upper surface of the base member when the actuator is in thelocked position, thereby preventing relative rotation between theactuator and the base member.
 2. The device of claim 1, wherein the atleast one biasing member comprises first and second biasing members,each of the first and second biasing members extending between the basemember and the rack and biasing the rack and support member into theneutral position from substantially opposite directions.
 3. The deviceof claim 2, wherein the first and second biasing members comprise firstand second compression springs located between the rack and the basemember, wherein the first spring biases the rack in a first directionand the second spring biases the rack in a substantially opposite seconddirection.
 4. The device of claim 3, wherein the compression springs arepartially housed within the rack, wherein a first end of each spring isengaged with an internal surface of the rack and a second end of eachspring is engaged with the base member.
 5. The device of claim 3,wherein the base member is provided with first and second adjustmentmembers which can adjust the biasing force of the first and secondsprings, respectively.
 6. The device of claim 1, further comprising apinion shaft which is rotatably supported on the base member androtatably coupled to the pinion, wherein the pinion shaft has a secondlocking element at one end thereof, the first and second lockingelements engaging to prevent rotation of the pinion when the actuator isin the locked position.
 7. The device of claim 1, wherein at least oneof the base member and actuator includes one or more magnets, eachmagnet attracting the actuator towards the locked or unlocked positionwhen the actuator is within a predetermined distance of the base member.8. The device of claim 7, wherein the base member and first and secondactuator buttons include magnets such that each button is attracted tothe base member when it comes within a predetermined distance of thebase member.
 9. The device of claim 1, wherein the locking mechanismfurther comprises a cover member having an opening through which thesupport member extends, the cover member enclosing the locking mechanismand engaging the first and second actuator buttons such that axialmovement of the cover member will result in a corresponding movement ofthe actuator.
 10. The device of claim 9, further comprising at least oneflexible covering located between the pinion and the opening in thecover member so as to prevent ingress of contaminants into the device.11. The device of claim 1, wherein the base member includes a quickrelease coupling for connecting the device to the wearer.
 12. Aprosthetic limb comprising the wrist device of claim
 1. 13. A wristdevice for a prosthetic limb, the device comprising: a base memberconnectable to the wearer of the device; a support member connectable tothe limb, the support member being pivotably connected to the basemember such that the support member can pivot about a pivot axisrelative to the base member; a damping mechanism located between thebase member and the support member, the damping mechanism comprising: apinion connected to the support member and rotatable about the pivotaxis relative to the base member; a rack engaged with the pinion suchthat rotational motion of the pinion causes a linear motion of the rack;and at least one biasing member extending between the base member andthe rack, the biasing member biasing the rack and support member into aneutral position; and a locking mechanism for selectively preventingmovement of the support member relative to the base member, the lockingmechanism comprising an actuator which is moveable between locked andunlocked positions, the actuator including a first locking element whichprevents movement of the pinion when the actuator is in the lockedposition, wherein the actuator comprises an actuator shaft extendingthrough the pinion along the pivot axis, and first and second actuatorbuttons at opposite ends of the shaft, wherein the actuator can moveaxially along the pivot axis relative to the base member and pinion whenthe first or second button is pressed, one of the first and secondbuttons incorporating the first locking element, and wherein the buttonwhich incorporates the first locking element has a bottom surface whichengages an upper surface of the base member when the actuator is in thelocked position, thereby preventing relative rotation between theactuator and the base member.